1. Field of the Invention
The present invention relates to a diversity antenna communication system to be mounted on and used in conjunction with a mobile body. More specifically, the invention relates to a diversity antenna communication system having two or more receiving or transmitting systems with different propagation paths from one another in which a received signal from one of the receiving systems having a higher reception level than another is selected for use to reduce influences of deep fading.
2. Description of Prior Art
In a signal communication between mobile bodies (mobile stations) which is carried out mainly with radiowaves modulated with digital data, a propagation path on the ground is a multipass type propagation path, and therefore the envelope and phase of a radio wave fluctuate randomly in accordance with the Rayleigh's distribution law and the uniform distribution law, respectively, (i.e., the radio wave suffers from fading). In order to reduce such fading, there is a diversity receiving antenna communication system in which the receiving system is changed over from one receiving antenna to another.
Conventional diversity receiving antenna communication systems are used primarily on a mobile body, and, for example, diversity systems using space, polarization, arrival angles, time and frequency have been utilized. Of those systems, a diversity receiving antenna communication system using an arrival angle is best suited for being mounted on a ground mobile station (such as a vehicle, or the like) because of its compactness and its ability to reduce fading in a mobile body which remarkably changes a traveling direction thereof.
A diversity signal receiving apparatus employed in the above-described diversity antenna communication system is as shown in FIG. 1. That is, two signal receiving antennas 1' provided for different signal propagation paths are connected to two distributors 2', respectively. One of the distributors 2' is connected to a signal receiver 3' and to a level monitor circuit 5'; whereas the other distributor 2' is connected to another level monitor circuit 5' and to a dummy resistor 4' equivalent in impedance to the signal receiver 3'. The two level monitor circuits 5' are connected to a level comparison circuit 6'.
The distributors 2' operate to distribute signals received through the receiving antennas 1' to loads (the receiver 3' and the dummy resistor 4') and to the level monitor circuits 5', respectively. The level monitor circuits 5' monitor the levels of the signal received through the distributors 2' (hereinafter referred to as "reception levels", when applicable). The level comparison circuit 6' compares the reception levels monitored by the level monitor circuits 5' with each other for detection of the reversal of the reception levels, so that in the signal receiving system providing a higher reception level the distributor is connected to the receiver, and in the preceding signal receiving system the distributor is connected to the dummy resistor.
FIG. 2 is a diagram for explaining reception levels in the conventional diversity signal receiving apparatus, and its antenna switching timing. More specifically, FIG. 2(A) shows an electric field strength at a signal receiving point, FIG. 2(B) shows the antenna switching timing, and FIG. 2(C) shows relationship between received data blocks and the antenna switching timing.
The two signal receiving antennas receive a radio wave from a signal transmitting station, to provided two reception levels (indicated by the thin solid line (a) and the broken line (b) in FIG. 2(A)). When the reception levels are reversed (as indicated by the broken line (d) in FIG. 2(B)), the signal receiving antenna selected previously is switched over to the receiving antenna providing the highest reception level, which provides a diversity field strength (as indicated by the heavy solid line (c) in FIG. 2(A)).
Thus, the signal can be received through the signal receiving system providing the highest reception level at all times. Therefore, the effect of deep fading can be minimized.
However, when the communication system switches one receiving antenna to the other, the phases before and after the change-over action are not continuous because of the difference in propagation paths. Further, although data which are divided into blocks of data signals, such as a synchronizing word, a control word, a talking word, and so on are transmitted continuously, the continuity of data, and moreover, the data itself are lost at the time of the change-over action as shown in FIG. 2(C). Therefore, to provide the lost data, it is necessary to retransmit.
In addition, in the case where the difference in level between the input signals received by the signal receiving system are small, the frequency of switching the signal receiving systems is increased, and accordingly the probability that the data is lost by the switching of the signal receiving systems is increased.
This disadvantage will be described in detail with reference to FIG. 3. In the case where the difference in level between the input (indicated by the solid line in FIG. 3(A)) received by a signal receiving system A and that (indicated by the broken line in FIG. 3(A)) received by a signal receiving system B is small (hereinafter referred to as "having substantially the same level", when applicable), the frequency of reversal of the reception levels is increased. A reception level determining unit detects such a frequent level reversal to output a signal receiving system switching signals (as shown in FIGS. 3(B) and 3(C)). Thus, the signal receiving systems are frequently switched over to each other, with the result that the data is cut and lost during the switching operation.
Furthermore, there is a problem that data transmitted by a mobile body is not transmitted to the called station. In order to transmit signals from a mobile body, a non-directional mono-pole antenna is used. With the non-directional antenna, data is not transmitted at some times because the position of the mobile body is changed frequently and therefore the reception level transmitted from the called station changes frequently.
For a single communication, signals are transmitted and received between fixed stations through directional diversity antennas. This is to separate an aimed station from a plurality of stations, thereby to perform selective communications.